Issue #21059: Temporary measure to make the Windows buildbots useful again.

Hopefully.

Tools/buildbot/test-amd64.bat

 @rem Used by the buildbot "test" step.
+
+rem The following line should be removed before #20035 is closed
+set TCL_LIBRARY=%CD%\..\tcltk64\lib\tcl8.6
 cd PCbuild
 call rt.bat -d -q -x64 -uall -rwW -n --timeout=3600 %1 %2 %3 %4 %5 %6 %7 %8 %9

Tools/buildbot/test.bat

 @rem Used by the buildbot "test" step.
+
+rem The following line should be removed before #20035 is closed
+set TCL_LIBRARY=%CD%\..\tcltk\lib\tcl8.6
 cd PCbuild
 call rt.bat -d -q -uall -rwW -n --timeout=3600 %1 %2 %3 %4 %5 %6 %7 %8 %9